/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.base.Function;
import com.google.common.collect.MapMakerInternalMap.InternalEntry;
import java.util.concurrent.ConcurrentMap;

/**
 * Contains static methods pertaining to instances of {@link Interner}.
 *
 * @author Kevin Bourrillion
 * @since 3.0
 */
@Beta
@GwtIncompatible
public final class Interners {
    private Interners() {}

    /**
     * Builder for {@link Interner} instances.
     *
     * @since 21.0
     */
    public static class InternerBuilder {
        private final MapMaker mapMaker = new MapMaker();
        private boolean strong = true;

        private InternerBuilder() {}

        /**
         * Instructs the {@link InternerBuilder} to build a strong interner.
         *
         * @see Interners#newStrongInterner()
         */
        public InternerBuilder strong() {
            this.strong = true;
            return this;
        }

        /**
         * Instructs the {@link InternerBuilder} to build a weak interner.
         *
         * @see Interners#newWeakInterner()
         */
        @GwtIncompatible("java.lang.ref.WeakReference")
        public InternerBuilder weak() {
            this.strong = false;
            return this;
        }

        /**
         * Sets the concurrency level that will be used by the to-be-built {@link Interner}.
         *
         * @see MapMaker#concurrencyLevel(int)
         */
        public InternerBuilder concurrencyLevel(int concurrencyLevel) {
            this.mapMaker.concurrencyLevel(concurrencyLevel);
            return this;
        }

        public <E> Interner<E> build() {
            return strong ? new StrongInterner<E>(mapMaker) : new WeakInterner<E>(mapMaker);
        }
    }

    /** Returns a fresh {@link InternerBuilder} instance. */
    public static InternerBuilder newBuilder() {
        return new InternerBuilder();
    }

    /**
     * Returns a new thread-safe interner which retains a strong reference to each instance it has
     * interned, thus preventing these instances from being garbage-collected. If this retention is
     * acceptable, this implementation may perform better than {@link #newWeakInterner}.
     */
    public static <E> Interner<E> newStrongInterner() {
        return newBuilder().strong().build();
    }

    /**
     * Returns a new thread-safe interner which retains a weak reference to each instance it has
     * interned, and so does not prevent these instances from being garbage-collected. This most
     * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative
     * when the memory usage of that implementation is unacceptable.
     */
    @GwtIncompatible("java.lang.ref.WeakReference")
    public static <E> Interner<E> newWeakInterner() {
        return newBuilder().weak().build();
    }

    @VisibleForTesting
    static final class StrongInterner<E> implements Interner<E> {
        @VisibleForTesting
        final ConcurrentMap<E, E> map;

        private StrongInterner(MapMaker mapMaker) {
            this.map = mapMaker.makeMap();
        }

        @Override
        public E intern(E sample) {
            E canonical = map.putIfAbsent(checkNotNull(sample), sample);
            return (canonical == null) ? sample : canonical;
        }
    }

    @VisibleForTesting
    static final class WeakInterner<E> implements Interner<E> {
        // MapMaker is our friend, we know about this type
        @VisibleForTesting
        final MapMakerInternalMap<E, Dummy, ?, ?> map;

        private WeakInterner(MapMaker mapMaker) {
            this.map = mapMaker.weakKeys().keyEquivalence(Equivalence.equals()).makeCustomMap();
        }

        @Override
        public E intern(E sample) {
            while (true) {
                // trying to read the canonical...
                InternalEntry<E, Dummy, ?> entry = map.getEntry(sample);
                if (entry != null) {
                    E canonical = entry.getKey();
                    if (canonical != null) { // only matters if weak/soft keys are used
                        return canonical;
                    }
                }

                // didn't see it, trying to put it instead...
                Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
                if (sneaky == null) {
                    return sample;
                } else {
                    /*
                     * Someone beat us to it! Trying again...
                     *
                     * Technically this loop not guaranteed to terminate, so theoretically
                     * (extremely unlikely) this thread might starve, but even then, there is always
                     * going to be another thread doing progress here.
                     */
                }
            }
        }

        private enum Dummy {
            VALUE
        }
    }

    /**
     * Returns a function that delegates to the {@link Interner#intern} method of the given
     * interner.
     *
     * @since 8.0
     */
    public static <E> Function<E, E> asFunction(Interner<E> interner) {
        return new InternerFunction<E>(checkNotNull(interner));
    }

    private static class InternerFunction<E> implements Function<E, E> {

        private final Interner<E> interner;

        public InternerFunction(Interner<E> interner) {
            this.interner = interner;
        }

        @Override
        public E apply(E input) {
            return interner.intern(input);
        }

        @Override
        public int hashCode() {
            return interner.hashCode();
        }

        @Override
        public boolean equals(Object other) {
            if (other instanceof InternerFunction) {
                InternerFunction<?> that = (InternerFunction<?>) other;
                return interner.equals(that.interner);
            }

            return false;
        }
    }
}
